It has now been urged to efficiently produce polycrystalline silicon of a high purity accompanying the use of polycrystalline silicon in a variety of fields as a starting material of semiconductors and cells for solar generation that are expected to be further developed and demanded in the future.
As a conventional method of producing polycrystalline silicon, there can be exemplified the Siemens' method according to which the surface of a silicon rod arranged, for example, in a bell jar is heated, and a starting gas for precipitating silicon containing chlorosilanes such as trichlorosilane (SiHCl3) or monosilane (SiH4) and a reducing gas such as hydrogen or the like is brought into contact with the surface of the silicon rod to precipitate the polycrystalline silicon.
The Siemens' method has a feature in that it is capable of obtaining silicon of a high purity accompanied, however, by a problem in that since the precipitation is conducted batchwise, a very complex procedure must be conducted such as installing the silicon rod which plays a principal role, heating by flowing an electric current, precipitation, cooling, take out and washing the bell jar.
As a method of efficiently producing silicon and as an apparatus therefor, that may solve the above problem, there have been proposed a method of producing polycrystalline silicon by feeding a starting gas for precipitating silicon into a cylindrical reaction vessel heated at a temperature lower than a melting point of silicon to precipitate silicon, heating the inner surfaces of the cylindrical reaction vessel at a temperature higher than the melting point of silicon, and melting part or whole of the precipitated silicon so that the precipitated silicon falls and is recovered, and an apparatus for producing silicon used for the above method (see patent document 1).
FIG. 6 and FIG. 7 illustrate a conventional apparatus for producing silicon, wherein FIG. 6 is a sectional view of the apparatus for producing silicon and FIG. 7 is a sectional view illustrating, on an enlarged scale, a portion of the reaction unit of the apparatus for producing silicon.
The apparatus 51 for producing silicon includes a reaction unit 52 positioned on the upper side and a recovery unit 53 positioned on the lower side. At the central portion of a ceiling wall of a reaction vessel body 54, a gas feed pipe 55 is provided to feed chlorosilanes and hydrogen which are the starting materials of silicon. A reaction tube 56 is arranged surrounding the outer circumference of the gas feed pipe 55 maintaining a distance. A high-frequency heating coil 61 is wound surrounding the outer circumference of the reaction tube 56 with a heat insulating material 64 interposed therebetween. A partitioning wall 65 is arranged under the reaction tube 56 maintaining a gap. The heat insulating material 64 is made of a carbon graphite having resistance against the heat.
A recovery vessel 58 is placed on a support floor wall 59 of a recovery unit 53 that is positioned at the lower end portion of the reaction vessel body 54. The recovery unit is of a structure that is disassembled to take out the recovery vessel 58. A cooling jacket 60 is arranged in the support floor wall 59. A feed port and a drain port of cooling means that is not shown are connected to the cooling jacket 60, and cooling water is circulated therein. A gas exhaust pipe 63 is arranged between the reaction unit 52 and the recovery unit 53, the gas exhaust pipe 63 being connected to a gas treatment facility on the downstream side.
In the apparatus 51 for producing silicon of the above structure, if a voltage is applied to the high-frequency heating coil 61, the reaction tube 56 is heated by eddy currents due to a high frequency of the high-frequency heating coil 61, and the inner surface of the reaction tube 56 is heated at a temperature in excess of the melting point of silicon. Chlorosilanes and hydrogen are fed from the gas feed pipe 55. These gases come in contact with the inner surface of the reaction tube 56, and silicon is precipitated in a molten state. The silicon solution precipitated in the molten state falls along the lower end portion of the reaction tube 56, i.e., flows down from the opening at the lower end portion of the reaction tube 56 and is recovered in the recovery vessel 58 positioned just underneath.
The silicon recovered in the recovery vessel 58 is cooled by the cooling water in the cooling jacket 60 before the recovery vessel 58 is taken out; i.e., the silicon is taken out of the reaction vessel body 54 together with the recovery vessel 58 in the cooled and solidified state.